This invention relates generally to yarns, and more particularly, is directed to a conductive yarn and a method and apparatus for making the same.
In many instances, it is desirable and often necessary to provide fabric which is conductive or which at least has some portions thereof that are conductive. For example, such conductive fabric can be used in transcutaneous transducer garments sold by Bio-Stimu Trend Corp. of Opa Locka, Fla. In such garments, electrodes are attached to the garments to stimulate the muscles of a patient. In the past, such electrodes have been applied directly to the skin of the patient with a wire attached from each electrode to a source of electric potential. However, when the electrodes are constructed into the garments, portions of the garment can be made of a conductive yarn so as to connect the electrodes to the source of electric potential without the numerous loose wires and the like previously used.
As a result, patient mobility is not restricted by cables, wires or tapes. There is also no separation or disconnection of the electrodes from the body parts by movement or after prolonged wear. Still further, several garments may be worn and simultaneously supplied with current. Also, because the electrodes are built into the garments, they do not require adhesives thereby eliminating certain types of allergic skin reactions, hair removal is not required, skin burns are reduced and problems with skin perspiration and oily skin are effectively eliminated.
Another use for fabric made of a conductive yarn is as a lightning strike arrester, for example, in any airborne vehicle. Still another use is in a conductive wrist band as disclosed in U.S. Patent Application Ser. No. 794,755, filed Nov. 4, 1985, now Pat. No. 4,654,748, by John J. M. Rees, entitled Conductive Wrist Band, the entire disclosure of which is incorporated herein. Still, other examples are in use on medical garments, garments for an antistatic clean room, and as an antistatic sewing thread.
Of course, in order for such yarn to be conductive, the yarn must include a conductive material. In this regard, Bekaert NVSA sells a metal thread consisting only of stainless steel under the trademarks "Bekinox VS" and "Bekinox VN". However, such threads generally have little or no stretch and break easily. Therefore, the utility of such threads is extremely limited.
Bekaert NVSA also sells a spun yarn including chopped up fibers of intertwined polyester and metal under the trademarks "Bekitex L80/1" and "Bekitex BK50/3". Because a spun yarn is used wherein the polyester and metal threads are chopped up and intertwined, there is no continuous metal along the thread length and therefore, the conductivity throughout the fiber is not continuous. Further, there is an insufficient shock absorbing quality of the polyester yarn in such arrangement because of the chopped up nature of the spun yarn. Still further, because of the spun yarn, shedding occurs, resulting in a loss of stainless steel fibers in the Bekaert material due to washing, stretching and the like. As a result, the final product is weaker and loses some of its original desirable properties. More importantly, when using such a conductive yarn in garments, for example, in an electrical clean room, contamination of semiconductors and other electrical components may occur due to shedding of the stainless steel fibers. Still further, in order to produce the Bekaert material, multiple steps are required because of the nature of the spun yarn. Most importantly, however, there is not a high conductivity and the fiber is not consistent and uniform throughout its length that there may be faults in the materials, such as slubs and the like. It is also important that such materials be launderable and, as aforesaid, this results in a loss of some stainless steel fibers with the Bekaert Bekitex material.
A high conductivity graphite material with electrically conductive filaments wrapped around the filaments is also known from U.S. Pat. No. 4,590,122, assigned to Fiberite Corp. of Winona, Minn. In this patent, a metal thread is twisted about a carbon thread.
However, another important property that is needed in conductive yarns is stretchability. With the materal of this U.S. Patent, since carbon has a high modulus of elasticity, the material has a low stretchability. For example, for a conductive yarn to be used in the applications described above, it is preferable that the percent elongation at break be at least about 3%, and preferably within the range of 10 to 15%. The average percent elongation at break for carbon is usually 1.5% or less.
Still further, carbon is a difficult material to work with since it is not very flexible. Therefore, carbon tends to shed and the fibers thereof tend to break. Also, because of the smooth surface of carbon, metal threads wrapped thereabout tend to slip on such smooth surface and group together in spaced apart bunches. Stil further, because carbon fibers tend to break when flexed, the material of U.S. Pat. No. 4,590,122 is generally not launderable.